High-voltage, direct current (HVDC) electric power transmission systems use direct current for the bulk transmission of electrical power, in contrast with the more common alternating current (AC) systems. HVDC systems may provide the ability to transmit large amounts of power over long distances with lower capital costs and lower losses than AC systems. Thus, HVDC systems may allow efficient use of energy sources remote from load centers. However, HVDC systems may present problems not present in AC systems. For example, unlike AC systems, HVDC system circuit breakers may be difficult to build because some mechanism must typically be included in the circuit breaker to force current to zero, otherwise arcing and contact wear would be too great to allow reliable switching.